Suggestions are in bold and underlined type, edits are struck through.

SCP-XXXX is to have Three guards are to be stationed at the sole entrance of SCP-XXXX at all times,

Try to find ways to not use "SCP-XXXX" two times in the same sentence, it makes for bad writing. The phrase "at all times" appears frequently in special containment procedures, but 98% of the time is unnecessary; we already know the containment procedures are to be followed at all times, that is the point of containment.

and they should turn around any civilian trying to enter SCP-XXXX telling them it's private property.

The clinical tone of this needs a lot of help. For example, "turn around" is not a clinically appropriate phrase. Let's see how this can be reworded:
"Three guards are to be stationed at the sole entrance of SCP-XXXX and are to assume the guise that they are protecting private property, diverting any civilian attempts to enter."
This is still not great, but it is much better.

An 8 foot tall fence of conventional materials has been erected around the border of SCP-XXXX and should be checked for damage monthly.

The Foundation uses the metric system, so measuring in feet is never appropriate. This would be the point at which you would lose all remaining readers to a downvote.

Should SCP-XXXX-4, or less likely SCP-XXXX-1, SCP-XXXX-2, or SCP-XXXX-3 attempt to leave SCP-XXXX(insert a comma here) the current on site director should be notified immediately.

It's nice to know that SCP-XXXX-1 through -3 are less likely to attempt to leave, but that doesn't have any relevance with respect to notifying the site director, so we don't need that here.

The farmhouse appears to be in poor condition, seemingly almost uninhabitable.

Saying that something is almost uninhabitable and in poor condition are pretty similar, so pick one or the other. Instead, take the opportunity to tell us how it is in poor condition.

It is of standard size for an rural american house

"American" should be capitalized. Rural homes in America do not have a standardized size. You probably need to specify the dimensions of the house using meters.

Other than the farmhouse there is A very nearby barn of similar condition rests beside a cornfield.

How big is the barn? How big is the cornfield?

The only way to access SCP-XXXX is the roughlya 4.8 km driveway which links to the nearby town [REDACTED].

If it links to another town is it really a driveway? Or is it just a road? Not sure.

While it doesn't go onto the driveway it does surround the rest of the property, taking up a few square kilometers.

This is the first time we are told anything about the size of the actual property (it needs to be specified much earlier in the article), even though it's kinda just mentioned haphazardly here. This sentence doesn't really tell us much, just that the fog doesn't cover the driveway/road. I would remove it from the article completely.

Any attempt at Venturing into the fog will result in at first just a loss of visual contact with anyone standing outside of fog, but then all communication (including cell phones and radio devices) is completely lost.

It's understood that being in a thick fog will prevent someone from seeing other people standing outside of the fog. If the fog is thick enough, which I imagine it is in this case, you may not even see someone who is standing in the fog with you. You already said "a loss of…" so don't need to say "is completely lost" again.

It is presumed that at this point the person has expired as they never return from this.

It's bad logic to assume that just because we lose communications with someone that they are dead. I know that is an attractive existential commentary on the dependence of our generation on electronic devices, but the Foundation has no scientific or rational business assuming people die just because they either have lost comms with them and/or they haven't returned from the fog. Leave the effects of the fog at a loss of communications, and refrain from poorly speculating on behalf of the Foundation.

The more significant effects of SCP-XXXX occur when people enter the farmhouse, barn and cornfield. As all of these can trigger the summonings ofcause SCP-XXXX-1, SCP-XXXX-2, SCP-XXXX-3 or SCP-XXXX-4 to appear.

"can trigger the summonings of" is not of good clinical tone. In fact, "summonings" is not a valid word, unless you consult Wiktionary. The second sentence "As all of these…" is not a complete sentence, it is a phrase. It would be highlighted in Microsoft Word telling you as much. Fix this by combining the two sentences:
"The more significant effects of SCP-XXXX occur when people enter the farmhouse, barn and cornfield; all of these can cause SCP-XXXX-1, SCP-XXXX-2, SCP-XXXX-3 or SCP-XXXX-4 to appear."

The needed conditions of this and descriptions of SCP-XXXX-1, 2, 3, and 4 will be documented bellow.

"The needed conditions of" is a really awkward phrase. Maybe "Requisite conditions for the appearance of SCP-XXXX…" is better. However, you've already told us what causes these to appear, "when people enter the farmhouse, barn, and cornfield." Additionally, we will understand that the SCP instances will be described when we are reading the description, so we don't need this road sign of a notice about what is about to come. I'd remove this sentence from the article completely.

SCP-XXXX-1
SCP-XXXX-1 Appears to be a caucasian woman of 1.9 meters in height.

You don't need the bolded header (this goes for all SCP-XXXX-1 through -4). "Appears" shouldn't be capitalized. "caucasian" should be capitalized.

Whenever a male between the ages of 18 and 80 who is in a heterosexual romantic relationship or who was emtionally hurt by a heterosexual romance moves into the property with the intent on living there (add a comma here) there is a chance of SCP-XXXX-1 "marking" them (add a period here). SCP-XXXX-1 does so by placing a ravens feather on their pillow before their first night sleeping in the master bedroom of the farmhouse.

"Emotionally" is misspelled. "ravens" should be "raven's".

they will occasionally see SCP-XXXX-1 around the farm house,

Previously, it was "farmhouse", one word. I don't really know if one way is more correct, just be consistent with it is all I (and we, probably) ask.

in the farmhouse, but neversave for in the basement or outside the house andIn such sightings, SCP-XXXX-1 normally disappears after a few seconds.

You've already said they see SCP-XXXX-1 in the house, no need to mention it doesn't happen outside the house.

They always describe it as beautiful, regal and other things along those lines.

This is another good example of the clinical tone being way off. It should probably read something more like: "Witnesses regularly use the words "beautiful", "regal", and other terms of flattery to describe SCP-XXXX-1."

As the males interest and eventual infatuation grows for SCP-XXXX-1 (add a comma here) it will appear more and more frequently to them, eventually interacting with them.

SCP-XXXX-2 appears to be a young caucasian male of 1.8 meters in height.

SCP-XXXX-2 wears very low quality clothing common fromto the late 19th century common for the impoverished of the time.

You have a habit in your writing to repeat something later in the sentence that you've already said in that sentence. You also repeat words in the same sentence ("common"). Be on the lookout for these bad habits. "For the impoverished of the time" doesn't make any sense I'm afraid. I can tell what you're trying to say though, so let me suggest:
"SCP-XXXX-2 wears derelict clothing common to the late 19th century."

SCP-XXXX-2 appears to be extremely dirty, to the point where is looks like it's gone years without cleaning itself.

Once again, this is information we've already received, first by your mentioning of impoverished (which is a good word by the way). We don't need this sentence at all and I would recommend removing it from the article completely.

The presence of SCP-XXXX-2 is often made apparentpreceded by the appearance of a multitude of rodents, rats and roaches being the most common.

The last part is unnecessary information and makes the sentence structure awkward. I'd remove it.

If the person tries to approach SCP-XXXX-2 it will run away deeper into the basement and vanish one it leaves the persons line of site.

"once" is misspelled.

The cornfield at the edge of SCP-XXXX is on its own a spacial anomaly presumably going on forever and never overlapping with the fog.

This should probably be said earlier in the description when the cornfield is first mentioned. This would work better because we get this information up front and we can start learning about SCP-XXXX-3 right away with no introductory material.

SCP-XXXX-3 is a humanoid that stands a roughly 2.4 meters and resides in the cornfield. It lacks any hair or facial features beyond a large mouth. SCP-XXXX-3 wears a black, hoodless friar robe and a short noose as a necklace.

"Hoodless" should be "hood-less".

No matter where the person is and no matter how brave they are The sight of SCP-XXXX-3 will cause intense fear and unease, panic if they are in the cornfield itselfregardless of the individual's usual disposition regarding stress.

People in the cornfield will attempt to run away from SCP-XXXX-3, but it always positions itself in a way that they will just run deeper into the cornfield.

Tone issues. How about: "Individuals who witness SCP-XXXX-3 while in the cornfield will subsequently be unable to escape the field. All attempts to escape will result in increased disorientation. Tracking of such individuals has suggested that they continuously run away from possible exits of the field." This is just a quick example of how the clinical tone probably should be in order to survive the main site…and that's only probably.

Eventually all contact will be lost with the person and they will be presumed dead.
Crossed this out for previously discussed reasons.

It wears semi(add a hyphen here)formal clothes

over it's

Should be "its". "It's" means "it is".

It is capable of leaving the barn, but only in pursuit of people who left the barn (change this to a period). Once SCP-XXXX-4 has terminated the individual, it will crumble into ashes and reappear in the barn if there are more people in it.

So concerning SCP-XXXX-1, what happens if the individual doesn't sleep in the master bedroom of the farmhouse? What if the guy sleeps on the couch? Is there still a raven's feather? My point is that, while a poetic idea, it is just really out-of-the-blue that this entity maybe puts a feather on the pillow, but maybe not.

"Marked" males will first see SCP-XXXX-1 in their dreams of their first night.

I actually really like this. Honestly, it is the only redeeming part of the article so far, but I do say, nice idea. I like the idea of SCP-XXXX-1 appearing more frequently as the marked males like her more.

There has only been one instance of SCP-XXXX-2 attacking someone entering the basement.

This is interesting and obviously goes against the picture so far painted for SCP-XXXX-2. I feel like it needs some narrative backing. Otherwise, it just does all the describing you've done so far a disservice.

SCP-XXXX-3 is a humanoid that stands a roughly 2.4 meters and resides in the cornfield. It lacks any hair or facial features beyond a large mouth. SCP-XXXX-3 wears a black, hoodless friar robe and a short noose as a necklace.

“A” should be “at”. Otherwise this is also good conceptual stuff.

If they do then the entire barn will burst into flames and SCP-XXXX-4 will materialize and attack them.

You have already mentioned that SCP-XXXX-4 will materialize, no need to say it again. You could say "After SCP-XXXX-4 materializes, it will attack the individual and the barn will burst into flames."

Generally speaking, this SCP tries too hard to do too much. I appreciate the multi-faceted approach to this, but it comes across as too many ideas pressed too tightly into too little space. Maybe just make a SCP about SCP-XXXX-1.

Overall, I'd say this article is maimed by spelling errors, grammatical mistakes, poor clinical tone, and weakly-developed ideas. In a very non-offensive way, I'd have to follow Zyn's lead on one of his reviews and ask your age. Successful articles typically require a college-level or even post-college level of writing. Writing on this site is very tough and the community very unforgiving, but don't give up, learn from the corrections here, and try again.

Alright, I've just finished going through all of the errors you've pointed out and I have to say that the article has been vastly improved and is far less messy.

As for your question for my age, i'm only 16 and to be honest very new to writing. But I have been doing creative work as an artist for a few years (which is probably where all the redeeming qualities of this SCP come from.)

And for the point you raised on weakly-developed ideas (such as SCP-XXXX-2 attacking someone) I am planning on writing addendums soon. I just felt that I should have stopped were I did to see how well the SCP itself would be received. (looking back now I can see the issues with that philosophy).

But I do want to thank you once more for going through the mess called my writing and giving me such helpful pointers.